for this i just screenshot a cube from 3DS Max for lighting direction ref and started scribbling ideas over it in photoshop by "carving" (painting) shapes in and out using light or dark. Its an interesting method that feels a bit like sculpting, while still being 2D. Now that I think about it, maybe I should try this approach more....hmmmm.....

the more you do it the easier it gets. The hardest part is learning not to overthink things. Start loose and just block things in, then start slowly going over your whole piece in upgrade passes until you're down to just an overall detail pass. Helps tons.